The present disclosure relates to an image reading device operable to read an original document and create image data from the read original document. The present disclosure also relates to an image forming apparatus with the image reading device.
Image reading devices generally include a light source and an image sensor. The light source irradiates a target to be read (an original document) with light. The reflected light from the target to be read is guided to the image sensor. The image reading device acquires image data based on an output of the image sensor. For example, a line sensor is provided as the image sensor in the image reading device. The read time (scan time) per line for which the image sensor reads one line of the target to be read is normally fixed. However, during reading of a sheet of original document, the read time per line may be changed to change the read rate. In this case, it is necessary to control, depending on the read rate, the amount of light applied to the original document within the read time per line. If not controlled, the amount of light received during reading of one line will vary from line to line even when the original document having a uniform concentration is read. Thus, the image data element read from each line will vary in concentration (pixel value) depending on the read rate, resulting in failure to correctly read the original document.
A description will be given of an example of an image reading device in relation to changes in read rate during reading of an identical original document. An image reading device can change the read rate by changing each of the scan time per line of a line sensor and the original conveyance rate of an original conveyance mechanism and controls, according to the scan time per line, the luminescence intensity of a light source for irradiating the original document with light. This image reading device compensates for variations in amount of light received by the line sensor due to changes in scan time per line, such as for the purpose of changing the read rate, with the control on the luminescence intensity of the light source.
Image reading devices, during reading of a sheet of original document, may increase and decrease the read time per line to change the read rate. For example, when an original document placed on an contact glass plate is read by moving a light source, a mirror for guiding reflected light to an image sensor, and so on, a certain time and a certain movement distance are necessary to increase the movement rate to a reference rate even if the light source and so on are moved at a constant rate. Furthermore, in order to increase the rate of acceleration of movement of the light source and so on, a motor having a large torque, for example, is necessary. Such a high-power motor meeting a desired capability is large and expensive and the mounting thereof on the image reading device is unrealistic. Instead of this, the rate of movement of the light source and so on is gradually accelerated to a reference read rate and, upon achievement of the reference rate, the light source is allowed to illuminate at a predetermined amount of light. Thus, image data obtained by reading of a sheet of original document is avoided from showing concentration differences (degradation of image quality) due to changes in read rate, so that reading can be performed even using a low-torque motor.
If the processing of image data obtained by reading an original document is delayed, a buffer built in the image reading device to temporarily store image data will overflow unless the rate of creation of image data is decreased. Therefore, in order to obviate the occurrence of overflow in the buffer, the read rate may be intentionally decreased. When the image data output (transfer) rate is recovered, the read rate may be increased back to the reference read rate.
As just described, there also exists such an image reading device which can read an image of an original document to be read by irradiating the original document with light from a light source while increasing and decreasing the read rate.